1. Field of the Invention
The invention relates to a constant on time buck converter, and more particularly, to a constant on time buck converter with small equivalent series resistance.
2. Description of the Related Art
A buck converter is a step-down DC to DC converter. Its design is similar to the step-up boost converter, and like the boost converter, it is a switched-mode power supply (SMPS) that uses two switches.
As well known in the Art, the simplest way to reduce a DC voltage is to use a voltage divider circuit. But voltage dividers waste energy since they operate by bleeding off excess voltage as heat. A buck converter, on the other hand, is not only a self-regulating DC to DC converter (output voltage varies with input voltage), but also may be remarkably efficient by wasting minimal energy. Thus, the buck converter is one of the most popular power supply converters used in many electronic devices.
FIG. 1 shows a conventional constant on time (COT) buck converter 100. The buck converter 100 comprises two switches SW1 and SW2, an indictor L1, a capacitor C1, a loading resistor RLOAD, an input volatge VIN, a reference voltage VREF, a comparator 103, a one shot 104, and an equivalent series resistance (ESR) of the capacitor C1. During the operation of the buck converter 100, the output voltage VO will be fed back to one terminal of the comparator 103. The comparator 103 compares the voltage of the output voltage VO with the reference voltage VREF, and outputs a corresponding control signal with a predetermined voltage so as to control the one shot 104 to turn on the switch SW1 for a constant time when the output voltage VO does not exceed the reference voltage VREF. Additionally, a constant voltage is output by adjusting the operating time of the switches. FIG. 2 is a figure showing the relationship between the output voltage VO, the reference voltage VREF, and the operating time of the switch SW1. As shown in FIG. 2, the switch SW1 is turned on for a constant time interval Ton according to the control signal outputted by the one shot 104 and turned off thereafter. The switch will be turned on again when the the output voltage VO falls below the reference voltage VREF. The operating time of the switch SW1 is controlled by the one shot and is proportional to the input voltage and the output voltage, for example TON=K×(VO/VIn), wherein K is an adjustable positive number.
Since the constant on time buck converter does not need feedback compensation, it may provide rapid response according to instantaneous change. Conventional constant on time buck converters may be limited by ESR resistance, which may be as large as tens of miliohms to hundreds of miliohms, for providing stable output voltage. Thus, the use of a ceramic capacitor with ESR resistance lower than 10 miliohms is not allowed. FIG. 3A is a figure showing the relationship between time t and the inductor current iL. FIG. 3B is a figure showing the relationships between time t and the voltages across the ESR VESR, wherein curves B1 to B3 respectively represents the measured VESR voltage curve as the ESR resistance is being increased. FIG. 3C is a figure showing the relationships between time t and the voltage across the capacitor VC, and FIG. 3D is a figure showing the relationships between time t and the output voltage VO, wherein VO=VC+VESR, and curves D1 to D3 respectively represents the measured VO voltage curve as the ESR resistance is being increased. As shown in FIG. 3A to FIG. 3D, the voltage across the ESR VESR and the inductor current iL are with the same phase, and the phase of the voltage across the capacitor VC is delayed with respect to the phase of the inductor current iL with a predetermined value due to the discharge of the capacitor C1. As the resistance of the ESR decreases, the output voltage VO will be dominated by the voltage across the capacitor VC so that the phase of the output voltage VO will be delayed with respect to the phase of the inductor current iL and the amplitude of the output voltage VO will be decreased. As such, system stability is accordingly decreased. On the other hand, as the resistance of the ESR increases, the output voltage VO will be dominated by the voltage across the ESR VESR so that the phase of the output voltage VO will be close to the phase of the inductor current iL and the amplitude of the output voltage VO will be increased. As such, system stability is accordingly increased. Although system stability increases as the resistance of the ESR increases, the output ripple voltage also increases according to the increasing resistance.
Thus, an improved design for achieving rapid response time and system stability with small equivalent series resistance by using a simple circuit without feedback compensation is needed.